You get ALLOT more than you pay for in this case. Most of them are pretty awesome! Just make sure you're getting aspherical multi-coated multi element filters and you'll be ecstatic with the results! I have quite a few. I keep picking up different ones for different filter sizes when I see them for like $20 ~ $40. All of mine so far (all no-brand) totally rock!

You get ALLOT more than you pay for in this case. Most of them are pretty awesome! Just make sure you're getting aspherical multi-coated multi element filters and you'll be ecstatic with the results! I have quite a few. I keep picking up different ones for different filter sizes when I see them for like $20 ~ $40. All of mine so far (all no-brand) totally rock!

I've tried these sorts of things before. They may not compare to a $1000 fisheye, but for $40 sometimes they work pretty good. Rather than order one, I'd find a camera store that stocks them, ask if I could take a few in-store test shots, and see if it would meet my quality needs. I've seen a wide range of quality between different models and brands.

Sometimes they even compete and favorably I might add, to $500~$1000 fisheyes! Remember they have to do allot less! There's is no engineering of barrel mechanics, there's no aperture blades, no bayonet mount, no focus ring or ability to focus, etc.. All that is dependent on the lens it attaches to. So all the manufacturers of these things need is multicoated achromatic or apochromatic lenses and a threaded tube. And when shopping that's all you need to look for - those three variables. Unfortunately, manufacturing, packaging, and marketing is cut to such a minimum that the lenses and packages often don't even say - so you have to look for yourself in many cases. Or just buy it and add it to your collection as I do and not worry about it till you get it home. If you're not hurting too much for money $20 to $40 a pop 3 or 4 times a year isn't anything.

It's easy to tell a multicoated lens. They have deep colored reflections and the glare incidence angle is very sharp. Single coated lenses have lighter colored reflections and the glare incidence angle is softer.

An achromatic lens has a front element with uneven or broken curvature across the face. It seems to almost flatten out near the outer edges. It's subtle so you have to spend a few moments looking at how reflections move across the face of the lens as you rock it around in your hand. This is also sometimes called an aspherical lens. It's almost always a doublet but that can be difficult to see.

An apochromat is more difficult to tell. It will have curvature like or identical to the achromat but additionally uses a special kind of glass the dispersion properties of which also fight (or correct) CA. If you hold the two side by side the apochromat will look kind of smoky - err, like smoked glass. It's still very clear and all but at certain angles the AC lens will look white bright and the APO lens will smoky-grey dark as you look through them.

After you see or learn to see, the differences for the first time detection becomes much easier.

If it's not multicoated don't waste you time. If it's not at least aspherical/achromatic don't waste your time. A wide angle achromat conversion lens is fairly beautiful! You won't have any complaints except for those ultra bright sky through tree-leafs shots where you'll see some moderate fringing - but not too bad.

An apochromatic (aka APO) lens corrects for spherical aberration at multiple wavelengths so leaf edges might appear just a tad on the soft side but there won't be any fringing worth mentioning. The APO wide/fish-eye converter is the best you're likely to find.

There usually won't be much of a price difference between all of these types however - unless one company offers several otherwise identical, kinds.

There are Superachromatic lenses too but I've never seen one in an converter attachment lens. It's often used (in 35mm SLR stuff) for IR specialty lenses. It usually has a triplet instead of a doublet as the front group.

PS: I've never seen a converter attachment made for 35mm SLRs that wasn't MC AC (Multicoated, Achromatic) although there are some for video cameras.